Page 1                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

     NAME
          fanotify_init - create and initialize fanotify group

     SYNOPSIS
          #include <fcntl.h>
          #include <sys/fanotify.h>

          int fanotify_init(unsigned int flags, unsigned int
          event_f_flags);

     DESCRIPTION
          For an overview of the fanotify API, see fanotify(7).

          fanotify_init() initializes a new fanotify group and returns
          a file descriptor for the event queue associated with the
          group.

          The file descriptor is used in calls to fanotify_mark(2) to
          specify the files, directories, mounts or filesystems for
          which fanotify events shall be created.  These events are
          received by reading from the file descriptor.  Some events
          are only informative, indicating that a file has been
          accessed.  Other events can be used to determine whether
          another application is permitted to access a file or direc-
          tory.  Permission to access filesystem objects is granted by
          writing to the file descriptor.

          Multiple programs may be using the fanotify interface at the
          same time to monitor the same files.

          In the current implementation, the number of fanotify groups
          per user is limited to 128.  This limit cannot be overrid-
          den.

          Calling fanotify_init() requires the CAP_SYS_ADMIN capabil-
          ity.  This constraint might be relaxed in future versions of
          the API.  Therefore, certain additional capability checks
          have been implemented as indicated below.

          The flags argument contains a multi-bit field defining the
          notification class of the listening application and further
          single bit fields specifying the behavior of the file
          descriptor.

          If multiple listeners for permission events exist, the noti-
          fication class is used to establish the sequence in which
          the listeners receive the events.

          Only one of the following notification classes may be speci-
          fied in flags:

     Page 2                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

          FAN_CLASS_PRE_CONTENT
               This value allows the receipt of events notifying that
               a file has been accessed and events for permission
               decisions if a file may be accessed.  It is intended
               for event listeners that need to access files before
               they contain their final data.  This notification class
               might be used by hierarchical storage managers, for
               example.

          FAN_CLASS_CONTENT
               This value allows the receipt of events notifying that
               a file has been accessed and events for permission
               decisions if a file may be accessed.  It is intended
               for event listeners that need to access files when they
               already contain their final content.  This notification
               class might be used by malware detection programs, for
               example.

          FAN_CLASS_NOTIF
               This is the default value.  It does not need to be
               specified.  This value only allows the receipt of
               events notifying that a file has been accessed.  Per-
               mission decisions before the file is accessed are not
               possible.

          Listeners with different notification classes will receive
          events in the order FAN_CLASS_PRE_CONTENT,
          FAN_CLASS_CONTENT, FAN_CLASS_NOTIF.  The order of notifica-
          tion for listeners in the same notification class is unde-
          fined.

          The following bits can additionally be set in flags:

          FAN_CLOEXEC
               Set the close-on-exec flag (FD_CLOEXEC) on the new file
               descriptor.  See the description of the O_CLOEXEC flag
               in open(2).

          FAN_NONBLOCK
               Enable the nonblocking flag (O_NONBLOCK) for the file
               descriptor.  Reading from the file descriptor will not
               block.  Instead, if no data is available, read(2) fails
               with the error EAGAIN.

          FAN_UNLIMITED_QUEUE
               Remove the limit of 16384 events for the event queue.
               Use of this flag requires the CAP_SYS_ADMIN capability.

          FAN_UNLIMITED_MARKS
               Remove the limit of 8192 marks.  Use of this flag
               requires the CAP_SYS_ADMIN capability.

     Page 3                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

          FAN_REPORT_TID (since Linux 4.20)
               Report thread ID (TID) instead of process ID (PID) in
               the pid field of the struct fanotify_event_metadata
               supplied to read(2) (see fanotify(7)).

          FAN_REPORT_FID (since Linux 5.1)
               This value allows the receipt of events which contain
               additional information about the underlying filesystem
               object correlated to an event.  An additional record of
               type FAN_EVENT_INFO_TYPE_FID encapsulates the informa-
               tion about the object and is included alongside the
               generic event metadata structure.  The file descriptor
               that is used to represent the object correlated to an
               event is instead substituted with a file handle.  It is
               intended for applications that may find the use of a
               file handle to identify an object more suitable than a
               file descriptor.  Additionally, it may be used for
               applications monitoring a directory or a filesystem
               that are interested in the directory entry modification
               events FAN_CREATE, FAN_DELETE, and FAN_MOVE, or in
               events such as FAN_ATTRIB, FAN_DELETE_SELF, and
               FAN_MOVE_SELF.  All the events above require an fan-
               otify group that identifies filesystem objects by file
               handles.  Note that for the directory entry modifica-
               tion events the reported file handle identifies the
               modified directory and not the created/deleted/moved
               child object.  The use of FAN_CLASS_CONTENT or
               FAN_CLASS_PRE_CONTENT is not permitted with this flag
               and will result in the error EINVAL.  See fanotify(7)
               for additional details.

          FAN_REPORT_DIR_FID (since Linux 5.9)
               Events for fanotify groups initialized with this flag
               will contain (see exceptions below) additional informa-
               tion about a directory object correlated to an event.
               An additional record of type FAN_EVENT_INFO_TYPE_DFID
               encapsulates the information about the directory object
               and is included alongside the generic event metadata
               structure.  For events that occur on a non-directory
               object, the additional structure includes a file handle
               that identifies the parent directory filesystem object.
               Note that there is no guarantee that the directory
               filesystem object will be found at the location
               described by the file handle information at the time
               the event is received.  When combined with the flag
               FAN_REPORT_FID, two records may be reported with events
               that occur on a non-directory object, one to identify
               the non-directory object itself and one to identify the
               parent directory object.  Note that in some cases, a
               filesystem object does not have a parent, for example,
               when an event occurs on an unlinked but open file.  In
               that case, with the FAN_REPORT_FID flag, the event will

     Page 4                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

               be reported with only one record to identify the non-
               directory object itself, because there is no directory
               associated with the event.  Without the FAN_REPORT_FID
               flag, no event will be reported.  See fanotify(7) for
               additional details.

          FAN_REPORT_NAME (since Linux 5.9)
               Events for fanotify groups initialized with this flag
               will contain additional information about the name of
               the directory entry correlated to an event.  This flag
               must be provided in conjunction with the flag
               FAN_REPORT_DIR_FID.  Providing this flag value without
               FAN_REPORT_DIR_FID will result in the error EINVAL.
               This flag may be combined with the flag FAN_REPORT_FID.
               An additional record of type
               FAN_EVENT_INFO_TYPE_DFID_NAME, which encapsulates the
               information about the directory entry, is included
               alongside the generic event metadata structure and sub-
               stitutes the additional information record of type
               FAN_EVENT_INFO_TYPE_DFID.  The additional record
               includes a file handle that identifies a directory
               filesystem object followed by a name that identifies an
               entry in that directory.  For the directory entry modi-
               fication events FAN_CREATE, FAN_DELETE, and FAN_MOVE,
               the reported name is that of the created/deleted/moved
               directory entry.  For other events that occur on a
               directory object, the reported file handle is that of
               the directory object itself and the reported name is
               '.'.  For other events that occur on a non-directory
               object, the reported file handle is that of the parent
               directory object and the reported name is the name of a
               directory entry where the object was located at the
               time of the event.  The rationale behind this logic is
               that the reported directory file handle can be passed
               to open_by_handle_at(2) to get an open directory file
               descriptor and that file descriptor along with the
               reported name can be used to call fstatat(2).  The same
               rule that applies to record type
               FAN_EVENT_INFO_TYPE_DFID also applies to record type
               FAN_EVENT_INFO_TYPE_DFID_NAME: if a non-directory
               object has no parent, either the event will not be
               reported or it will be reported without the directory
               entry information.  Note that there is no guarantee
               that the filesystem object will be found at the loca-
               tion described by the directory entry information at
               the time the event is received.  See fanotify(7) for
               additional details.

          FAN_REPORT_DFID_NAME
               This is a synonym for
               (FAN_REPORT_DIR_FID|FAN_REPORT_NAME).

     Page 5                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

          The event_f_flags argument defines the file status flags
          that will be set on the open file descriptions that are cre-
          ated for fanotify events.  For details of these flags, see
          the description of the flags values in open(2).
          event_f_flags includes a multi-bit field for the access
          mode.  This field can take the following values:

          O_RDONLY
               This value allows only read access.

          O_WRONLY
               This value allows only write access.

          O_RDWR
               This value allows read and write access.

          Additional bits can be set in event_f_flags. The most useful
          values are:

          O_LARGEFILE
               Enable support for files exceeding 2 GB.  Failing to
               set this flag will result in an EOVERFLOW error when
               trying to open a large file which is monitored by an
               fanotify group on a 32-bit system.

          O_CLOEXEC (since Linux 3.18)
               Enable the close-on-exec flag for the file descriptor.
               See the description of the O_CLOEXEC flag in open(2)
               for reasons why this may be useful.

          The following are also allowable: O_APPEND, O_DSYNC,
          O_NOATIME, O_NONBLOCK, and O_SYNC.  Specifying any other
          flag in event_f_flags yields the error EINVAL (but see
          BUGS).

     RETURN VALUE
          On success, fanotify_init() returns a new file descriptor.
          On error, -1 is returned, and errno is set to indicate the
          error.

     ERRORS
          EINVAL
               An invalid value was passed in flags or event_f_flags.
               FAN_ALL_INIT_FLAGS (deprecated since Linux kernel ver-
               sion 4.20) defines all allowable bits for flags.

          EMFILE
               The number of fanotify groups for this user exceeds
               128.

          EMFILE
               The per-process limit on the number of open file

     Page 6                        Linux             (printed 5/24/22)

     FANOTIFY_INIT(2)          (2020-11-01)           FANOTIFY_INIT(2)

               descriptors has been reached.

          ENOMEM
               The allocation of memory for the notification group
               failed.

          ENOSYS
               This kernel does not implement fanotify_init().  The
               fanotify API is available only if the kernel was con-
               figured with CONFIG_FANOTIFY.

          EPERM
               The operation is not permitted because the caller lacks
               the CAP_SYS_ADMIN capability.

     VERSIONS
          fanotify_init() was introduced in version 2.6.36 of the
          Linux kernel and enabled in version 2.6.37.

     CONFORMING TO
          This system call is Linux-specific.

     BUGS
          The following bug was present in Linux kernels before ver-
          sion 3.18:

          *  The O_CLOEXEC is ignored when passed in event_f_flags.

          The following bug was present in Linux kernels before ver-
          sion 3.14:

          *  The event_f_flags argument is not checked for invalid
             flags.  Flags that are intended only for internal use,
             such as FMODE_EXEC, can be set, and will consequently be
             set for the file descriptors returned when reading from
             the fanotify file descriptor.

     SEE ALSO
          fanotify_mark(2), fanotify(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 7                        Linux             (printed 5/24/22)